Val,
I shoot the AX100 with the Gates housing. I mostly use the flip down blue water filter or 10,000 lumen light at depth. When I do use white balance I have three favorites, wife's white fins, silver tank or white sand. I was told that for the Sony system a white card with "red" squares on it provides the best manual WB. It takes away the "sometimes redish" tint while using white balance. I'm satisfied so far; not toting a slate around. I Shoot 4K, 30 FPS as well. I set ISO to Auto and cap the upper limits. I'm sure you will get some great advice on this blog.